Why does a road ripple like a washboard? Here comes the science...
The good news is that the men in white coats have figured out why washboard roads develop on sand, gravel or any other loosely-surfaced roads. Above a certain speed, any linear force interacting with the surface causes the force to skip over the surface like a rock skipping over the water. The ripples are caused by the force alternately being "thrown off the surface" and then touching down again.